Recent progress in digital technology has made the widespread use of compressed digital video signals practical. Standardization has been very important in the development of common compression methods used in new services and products now possible. This allows the new services to interoperate and encourages the investment in integrated circuits to make the technology affordable.
MPEG (Moving Picture Experts Group) was founded in 1988 as a working group within ISO/IEC (International Standardization Organization/International Electrotechnical Commission) with the aim of defining standards for digital compression of audio-visual signals. MPEG's first standard, MPEG-1, was published in 1993 as ISO/IEC 11172. It is a three-part standard defining audio and video compression coding methods and a multiplexing system for interleaving audio and video data so that they can be played back together. MPEG-1 principally supports video coding up to about 1.5 Mbits/sec giving quality similar to VHS and stereo audio at 192 bit/sec. It is used in the CD-i (CD-interactive) and Video-CD systems for storing video and audio on CD-ROM.
During 1990, MPEG recognized the need for a second, related standard for coding video for broadcast formats at higher data rates. The MPEG-2 standard can code standard-definition television at bit rates from about 3 to 15 Mbits/sec and high-definition television at 15 to 30 Mbits/sec. MPEG-2 extends the stereo audio capabilities of MPEG-1 to multi-channel surround sound coding. MPEG-2 decoders can also decode MPEG-1 bitstreams.
The MPEG-2 standard is a generic video coding system supporting a diverse range of applications. Different algorithmic tools, developed for many applications, have been integrated into the full standard. To implement all the features of the standard in all decoders is unnecessarily complex and a waste of bandwidth, so a small number of subsets of the full standard, known as profiles and levels, have been defined. A profile is a subset of algorithmic tools and a level identifies a set of constraints on parameter values (such as picture size and bit rate). A decoder which supports a particular profile and level is only required to support the corresponding subset of the full standard and set of parameter constraints.
Television services in the United States broadcast video at a frame rate of slightly less than 30 Hz. Each frame consists of two interlaced fields, giving a field rate of slightly less than 60 Hz. The first field of each frame contains only the odd numbered lines of the frame (numbering the top frame line as line 1), and the second field contains only the even numbered lines of the frame. It is important to note that one interlaced frame contains fields from two instants in time.
In video systems other than television, non-interlaced video is commonplace (for example, most computers output non-interlaced video). In non-interlaced video, all the lines of a frame are sampled at the same time. Non-interlaced video is also termed progressively scanned or sequentially scanned video.
The red, green and blue (RGB) signals commonly used can be equivalently expressed as luminance (Y) and chrominance (UV) components. The chrominance bandwidth may be reduced relative to luminance without significantly affecting the picture quality. For standard definition video, CCIR (Consultative Committee for International Radio) 601 defines how the component (YUV) video signals can be sampled and digitized to form discrete pixels. The terms 4:2:2 and 4:2:0 are often used to describe the sampling structure of the digital picture. 4:2:2 means the chrominance is horizontally subsampled by a factor of two relative to the luminance; 4:2:0 means the chrominance is horizontally and vertically subsampled by a factor of two relative to the luminance.
Using 8 bits for each Y, U or V pixel, the uncompressed bit rate is over 150 Mbits/sec.
MPEG-2 is capable of compressing the bit rate of standard-definition 4:2:0 video down to about 3 to 15 Mbits/sec. At the lower bit rates in this range, the artifacts introduced by the MPEG-2 coding and decoding process are objectionable.
In order to improve encoded quality, an additional pre-filtering step is usually implemented at the input to the encoder. The purpose of the pre-filter is to reduce or eliminate any high frequency noise that may be present in the input video source. If present, such noise would degrade the performance of the video encoder because the encoder would waste available bits attempting to encode the noise.
In low bit rate applications, pre-filtering is also used to reduce the high frequency components of the source video to prevent blocking, ringing and mosquito noise in the decoder. Blocking noise is noise artifacts at the encoded block boundaries, while ringing and mosquito noise is usually due to excess high frequency components and is evident within the encoded blocks.
The pre-filter is usually implemented as a low pass filter. The amount of filtering is critical, as too much filtering will degrade the overall image quality.